JERSEY CITY -- The murder trial of Froylan Lopez -- the man accused of fatally stabbing 26-year-old Troy Brandon in 2015 -- began in earnest today with opening statements from both the state and defense teams.

While the state argues Brandon was fatally stabbed on Dec. 5, 2015 while coming to the aid of a man who had been assaulted in the street, the defense contends that Lopez thought he was protecting his friend from Brandon, whom he perceived to be the aggressor.

Hudson County Assistant Prosecutor Joseph Lanzot told jurors today that Brandon, watched as a large man knocked out a smaller man with a single punch to the head at Monticello and Storms avenues on the night of the incident. It had previously been incorrectly reported that the victim was a woman.

"Brandon could have just walked away, like many people do nowadays, and you will see in the video, many people actually did," Lanzot continued, saying that Brandon checked the man's condition, stood near him, dialed 911 and tried to flag down an police car.

Lanzot told the jury they will see security video showing that rather than assist, Lopez "took a knife and plunged it into the side of Troy Brandon, puncturing his lung and ultimately his heart." He said the video shows Brandon run a short distance before collapsing.

The prosecutor said Lopez, 30, fled.

But defense attorney Hope Mitchell told jurors "You are probably saying 'There's a video. This is going to be easy.' This is not going to be easy."

"I submit to you that when Froylan approached the scene, he saw his good friend on the ground in the street, bleeding and helpless and he perceived that," Mitchell said. "He approached Troy Brandon with that perception. In his mind, he was protecting his friend."

The prosecution's first witness, Jersey City Police Sgt. John Barone, said he was the first officer on the scene. He said he saw one man standing up and bleeding from the head. He said Brandon was on his back on the ground in pool of blood and he was unresponsive. Barone called an ambulance.

Barone said EMTs cut Brandon's clothing off and he saw a "pretty big puncture wound" below Brandon's left armpit.

Officials said the incident occurred at about 10:15 p.m. and Brandon was pronounced dead about 40 minutes later at the Jersey City Medical Center.

Lopez was arrested the next day after fleeing through a window and descending a fire escape, officials said. He is charged with murder and weapons offenses related to a knife, and faces 30 years to life in prison if convicted.

Lanzot said Lopez gave a statement to police putting himself at the scene of the crime that night. The defendant has pleaded not guilty.

Jury selection began Tuesday and nine women and five men were impaneled by about 12:15 p.m. today. Opening statements were heard after the lunch break.

After Barone, the Hudson County Prosecutor's Office homicide detective who recovered the security video was called. While on the stand, the jury was shown a portion of the video.

The trial will resume tomorrow morning with the detective still on the stand.